Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis.
Authors: Valberg S J, McKenzie E C, Eyrich L V, Shivers J, Barnes N E, Finno C J
Journal: Equine veterinary journal
Summary
# Editorial Summary: Myofibrillar Myopathy in Arabian Horses with Exertional Rhabdomyolysis Exertional rhabdomyolysis affects Arabian horses at notable rates, yet the underlying muscle pathology has never been systematically characterised in this breed. Valberg and colleagues examined middle gluteal muscle biopsies from Arabian endurance horses with a history of exertional rhabdomyolysis (prospective group, n=13), archived samples from horses previously diagnosed with various exertional myopathies (n=25), and healthy controls (n=14), using light microscopy, immunohistochemistry and electron microscopy to identify distinctive features. Central nucleation within mature muscle fibres was markedly elevated in affected horses (92% of prospective cases, 84% of archived cases) compared to controls (29%), whilst disrupted myofibrillar alignment, abnormal accumulation of desmin and α β-crystallin proteins, Z-disc degeneration and focal glycogen pools within damaged myofibrils were prominent in affected animals. Critically, many horses had been previously misdiagnosed with type 2 polysaccharide storage myopathy; the apparent glycogen accumulation reflected secondary pooling within disrupted myofibrillar architecture rather than primary glycogen storage disease. These findings have substantial implications for diagnosis and management strategies in Arabian endurance horses presenting with recurrent exertional episodes, suggesting that myofibrillar myopathy rather than metabolic glycogen storage may be the primary pathological process driving clinical signs in a significant subset of this population.
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Practical Takeaways
- •Arabian horses with recurrent exertional rhabdomyolysis may be misdiagnosed with polysaccharide storage myopathy; consider myofibrillar myopathy in differential diagnosis
- •Muscle biopsy with immunohistochemical staining for cytoskeletal proteins (desmin, α β-crystallin) and electron microscopy can help differentiate myofibrillar myopathy from glycogen storage disorders
- •Management strategies for myofibrillar myopathy may differ from polysaccharide storage myopathy, making accurate diagnosis important for treatment success in affected Arabian endurance horses
Key Findings
- •Centrally located myonuclei in mature myofibres were significantly more common in pro-ER (12/13) and retro-ER (21/25) horses compared to controls (4/14, P<0.05)
- •Disrupted myofibrillar alignment with large desmin and α β-crystallin positive cytoplasmic aggregates were found in significantly more pro-ER (12/13) and retro-ER (18/25) horses versus controls (3/14, P<0.05)
- •Electron microscopy revealed prominent Z-disc degeneration and focal myofibrillar disruption with β-glycogen particle accumulation in pro-ER samples
- •Many Arabian horses previously diagnosed with type 2 polysaccharide storage myopathy actually have myofibrillar myopathy with glycogen pools forming within disrupted myofibrils